var dustObj = function()
{
	this.x = [];
	this.y = [];
	this.amp = [];
	this.No = [];
	
	this.alpha;
}
dustObj.prototype.num = 20;
dustObj.prototype.init = function()
{
	for(var i=0;i<this.num;i++)
	{
		this.x[i] = Math.random() * canWidth;
		this.y[i] = Math.random() * canHeight;
		this.amp[i] = 20 + Math.random()*15;
		this.No[i] = Math.floor(Math.random()*7);// 归一 [0,7)
	}
	this.alpha = 0;
}
dustObj.prototype.draw = function()
{
	this.alpha += deltaTime * 0.0008;
	var l = Math.sin(this.alpha);
	for(var i=0;i<this.num;i++)
	{
		var no = this.No[i];
		ctx1.drawImage(dustPic[no],this.x[i] + this.amp[i]*l,this.y[i]);
	}
}
